In radar, telecommunications, and imaging systems a portion of the radiated and reflected electromagnetic energy is lost or attenuated in the transmission medium, e.g., in the atmosphere which contains absorbing gases and distributed water in the form of rain, sleet, snow or fog. Consequently, for the design, evaluation, and operation of such systems it is desirable to measure the attenuation caused by particular transmission conditions. A transmissometer is a device for measuring the transmission loss resulting from existing transmission conditions along a predetermined path through the transmission medium.
To measure attenuation, a known method is to radiate electromagnetic radiation from a transmitting antenna or source at a specified power along a predetermined path to a target from which the radiation is reflected back to a receiver near the transmitting antenna or source. The transmissometer system delivers a signal which is monotonically related to the power of the reflected signal which is received at the receiver. The target-reflected power must be detected independently of its phase, and the detection must not include reflections from other objects. The power reflected from the target may then be compared to the power reflected from the target under other, known transmission conditions, e.g., under lossless conditions, so that the attenuation under the existing transmission condition can then be determined.
Two types of such reflection transmissometers have been previously known. One is an ungated reflection transmissometer and the other is a radar transmissometer.
In ungated reflection transmissometers, electromagnetic energy is radiated as a continuous wave, either modulated or unmodulated, and the energy reflected from the target is detected and a signal monotonically related to its power is obtained. If the transmitted continuous wave is modulated, certain parts of the receiver circuitry may operate at the modulation frequency or its harmonics by using conventional frequency shifting. Additionally, radiation from other interfering sources at the transmitter frequency can be excluded from the receiver detector circuitry.
The problem with a continuous wave ungated transmissometer is that it is unable to distinguish between energy reflected from the intended target and energy reflected from other targets. Thus, energy received at the receiver, which was reflected from unknown or undesired targets is a source of error. For any given target, the received reflected power decreases as the reciprocal of the fourth power of the distance between target and transmissometer even in the absence of attenuation by the transmission medium. Therefore even small targets near the transmissometer produce large signals, and the sensitivity of ungated reflection transmissometers is limited by near-by reflections, e.g., by ground clutter. In addition, this system can only be used with a single target and thus multiple targets spaced at different ranges from the transmitting antenna can not be used. Therefore, previously existing continuous wave transmissometers do not allow the attenuation at multiple, different ranges to be measured concurrently.
However, continuous wave transmissometers do have the advantage that the radiated energy is radiated over a relatively long duty cycle, on the order of all or half of the entire observation time. This permits the use of a relatively low-power transmitter and eliminates the need for the precision range gating circuitry which is required in radar systems. As a result, ungated reflection transmissometers are simpler and may be less expensive than radar transmissometers.
Radar transmissometers have been used in order to distinguish one target from another and therefore exclude reflections from targets which are outside the design range interval or "range gate". They also permit the simultaneous use of multiple targets. With radar transmissometers, each desired target is identified and distinguished, as in conventional radar, by the time its reflection returns to the receiver. Thus, radar transmissometers can exclude some of the clutter reflections and permit the simultaneous measurement of attenuation at different ranges from several spaced targets. This is an important advantage of radar transmissometers over ungated transmissometers because of the nonuniform attenuation which can occur from changes in the medium with distance, such as is common in the atmosphere in the case of localized precipitation.
Radar transmissometers, however, like ungated continuous wave transmissometers still cannot discriminate against ground clutter or other significant reflections which occur within the range gate of the radar system.
Additionally, a radar system requires the radiation of a very high-power, electromagnetic wave over a short time interval. The pulse must be short to allow effective discrimination between the desired target and others, and the power must be high in order to cause sufficient reflected energy to be received at the receiver and detected during the observation time. Although discrimination against undesired targets and ground clutter can be improved by shortening the range gate time interval, this causes the ratio of observation time to total available time to be shortened, which in turn requires still higher transmitter power to achieve the required signal-to-noise ratio. In addition, of course, the radar transmissometer requires the complex and possibly expensive range gating circuitry.
These requirements can be avoided, at the cost of additional complexity, by the use of pulse-compression or "chirp" radar techniques. However, such systems require swept-frequency sources which are generally more expensive, and which are unavailable in many frequency ranges of interest.